Anesthetics are drugs which produce anesthesia, a condition of inability to appreciate sensation. Two types of anesthesia are usually recognized: local anesthesia, in which the anesthesia is confined to a portion of the body and under which the patient is conscious and general anesthesia, in which the anesthesia extends to the entire body and under which the patient is unconscious, in a state of muscular relaxation and insensibility to pain. The latter type is employed for most surgical operations.
The general anesthetics may be divided into three groups: inhalation anesthetics, intravenous anesthetics, and those which are used only to induce basal anesthesia. Inhalation anesthetics include the volatile liquids and gases.
Volatile liquids and gases are commonly used to induce anesthesia by progressively increasing the amount of volatile anesthetic in the inspired air and thus in the blood and brain. The administration of an anesthetic results in progressive depression of the central nervous system, which may be preceded by varying degrees of excitation. These drugs first depress the cerebral cortex and then the basal ganglia and cerebellum. At this level, a state of analgesia is induced with a reduction of perception to painful stimuli, useful in minor surgery. This stage is followed first by sensory and then motor paralysis of the functions of the spinal cord from below, upward. This is the level at which most major surgical procedures are carried out. If the administration of the anesthetic is continued, the medullary centers are involved, and death may result from paralysis of the respiratory and vasomotor centers.
The inhalation anesthetics are usually administered by one of two methods. In the open method; the liquid anesthetic is dropped on a cotton or guaze mask held over the patient's nose or mouth. Air is the diluent and no anesthetic machine is required. In the closed method; the gaseous or liquid anesthetic is contained in a special apparatus which, when attached to the patient's nose and mouth, constitutes a closed system. The patient is continually rebreathing the contents of the system. Provision is made for the removal of carbon dioxide with soda lime and the addition of oxygen as needed. Sometimes, a semiclosed method, a modification of the second method, is used. This method employs a closed method type of apparatus, but a valve on the mask permits ready expiration outside the system,
The gaseous anesthetic agents are vapors at ordinary room temperature and, in general, have boiling points at less than 20.degree. C. Consequently, they are confined under high pressure in cylinders and administered by the closed method with an anesthetic machine. The anesthetic machine controls the mixing of the anesthetic gas with oxygen or air so as to sustain respiration while maintaining the anesthetized state of the patient. These agents vary greatly in anesthetic potency, and their successful use often depends upon reliable control of the mixture of the anesthetic gas and oxygen or air supplied to the patient. Nitrous oxide is nonirritating and causes no untoward side actions if anoxia is avoided. But to avoid anoxia, close control must be maintained over the ratio of the anesthetic agent and the oxygen or air which is blended therewith. If the relative amount of oxygen is inadvertently reduced to below 20 percent thru a spent oxygen cylinder or defective oxygen supply, or human error anoxia is certain unless some means is present to detect this malfunction and sound an alarm. These considerations obtain for other gaseous anesthetics as well, such as cyclopropane and ethylene.
In addition, the margin for error in controlling the mixture of nitrous oxide and oxygen is small, inasmuch as high concentrations of nitrous oxide are required, little room is left in the mixture for oxygen. This may result in serious anoxia and tissue damage, especially to the central nervous system. For this reason, a reliable means to monitor the gas mixture is required for the proper use of nitrous oxide as an anesthetic.
Prior art systems for monitoring oxygen flow in anesthetic machines generally comprise mechanical actuators responsive to a reduction in the oxygen supply pressure below a predetermined minimum value. These actuators are comprised of many moving parts which are subject to wear, thereby impairing their reliability. These prior art actuators are, in addition, generally bulky and inconvenient to employ in portable anesthetic machines.